The present invention is directed to improving the service life of thermite welds for rails, and involves a special heat treatment of the existing thermite welds for improving their mechanical properties and thereby increasing their service life.
Thermite welding of rails is used throughout the world to join lengths of rails into continuous track work. There are other methods of welding rails, however, the thermite process has experienced wide utilization, due to its relative simplicity, portability of the equipment used, and its low cost. Throughout this century, thermite welds have afforded satisfactory service performance, relative to the service life of other types of welds and of the rail. The advent of increased axle loads in some localities and/or programs intended to extend the life of the rails have placed increased demand on thermite rail welds, which warrants programs aimed at improving the service life of the welds.
Since thermite welds are essentially steel castings, it has been proposed that the mechanical properties of the weld and, consequently, their service life, should be improved by a post-weld heat treatment. In heat treating experiments using samples sectioned from thermite welds, it has been found that the mechanical properties of the welds can be significantly improved by heating the weld into the austenite range and then air cooling, which is a conventional heat treatment known as normalizing. In normalizing, the main benefit is a refinement of the grain size. The weld metal in the rail head, however, may become softened during normalizing and this is undesirable for wear resistance. Further, it is known that thermite weld metal can be heat-treated without softening by heating it into the austenite range and then utilizing enhanced cooling to force the transformation from austenite to pearlite to occur at a lower temperature.
Therefore, it is known that samples sectioned from aluminothermic welds can be heat-treated under controlled conditions for producing weld metal with improved mechanical properties and required hardness by utilizing well-known metallurgical principles. The transfer of the known metallurgical principles from weld samples to the actual treatment of full size welds in existing track work is difficult to achieve and, at the present time, there are no known effective methods.